NASA shared a video on Twitter that Stephen Hawking recorded for astronauts Rick Mastracchio and Koichi Wakata on the International Space Station in 2014.

Remembering Stephen Hawking, a renowned physicist and ambassador of science. His theories unlocked a universe of possibilities that we & the world are exploring. May you keep flying like superman in microgravity, as you said to astronauts on @Space_Station in 2014 pic.twitter.com/FeR4fd2zZ5